This invention relates to the field of xerography and, particularly, to an improved corona generating device for imposing electrostatic charge on a xerographic plate or on a support surface.

In one mode of reproduction of copy by xerographic techniques, a uniform electrostatic charge is applied to a photoconductive surface and the charged surface is exposed to a light image, whereby an electrostatic latent image of the light image is formed on the xerographic plate. Thereafter, the latent image is developed with an electroscopic powder and the thus formed powder image is usually transferred to a support surface by superposing the support surface thereon and applying an electrostatic charge to the exposed area of the support surface, whereby the xerographic pow er image is electrostatically attracted and bonded theret The support surface is then separated from the xerographic plate and the xerographic powder image may then be fixed thereto by any of a variety of means.

In general, the electrostatic charging of the xerographic plate in preparation for the exposure step and the electro static charging of the support surface to effect transfer are accomplished by means of corona generating devices whereby electrostatic charge on the order of 500 to 606 volts is applied to the respective surface, in each instance. A form of corona generating device for this purpose is disclosed in Schafiert Patent No. 2,576,047, issued November 20, 1951, wherein a plurality of parallel wires are connected in series to a high voltage source and are supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the Wires are energized, corona is generated along the surface of the Wires and positive ions are caused to be deposited on the adjacent photoconductive surface or support surface, as the case may be. Suitable means are usually provided to effect relative movement of the surface to be charged and the corona genera ing device, whereby a uniform electrostatic charge is deposited.

Although corona generating devices of this type have certain limited application, they are not well suited to applications in which continuous uniform charging of a surface is to be effected. As is well known, the corona threshold potential and the corona current from a charged wire are functions of the thickness of the Wire such that the corona threshold increases with any increasing thickness of the wire and the corona current for any given potential decreases with any increasing thickness of the wire. In addition, the corona threshold potential and corona current are also affected directly by deposits of dust that may accumulate on the wire and by variations of movement and ionized conditions of the air sheath surrounding the wire. combined length of the wires employed, it is essential to maintain a corona potential on the wires at a value just above the threshold potentiais of the particular wires in order to keep, the current to the xerographic plate at a level such that it will not overcharge the plate. However, in operating at the corona threshold, minute difin such a device, in vie-w or" theferences in wire diameter, slight accumulations of dust on the wire, and variations in air current drastically affect the corona generating potential of the Wire and cause a non-uniform electrostatic charge to be deposited on the xerographic plate, thereby rendering such a device substantially unsuitable for xerographic plate charging.

Heretofore, it has been proposed to improve the effectiveness of such a charging device by inserting a biased wire shield between the corona wires and the xerographic plate. By this means, the corona wires may be energized at a potential well above the corona threshold potential thereof and the excess of corona current over that required for proper charging of the plate is drained ofi by the biased shield. Although devices of this type are highly elfective for xerographic plate charging and are currently in wide commercial use, they are expensive to manufacture and are difficult to maintain in proper operating adjustment.

The principal object of the present invention is to improve the construction of corona generating devices where by a uniform electrostatic charge may be deposited on a xerographic plate or other surface. A further object is to provide such a corona generating device that is of simple and rugged construction and is economical to manufacture and to maintain. A further object is to provide a corona generating device of such construction that is particularly suitable for continuous operation in automatic machines.

These and other objects of the invention are attained by employing a single corona wire that is connected to a high voltage source and is insulated from but supported within a conductive shield that substantially surrounds the wire but is provided with a longitudinal slit to permit positive ions generated by the wire to be emitted from the assembly. This device is supported in closely spaced relation to the xerographic plate that is to be charged, in the manner of the charging devices mentioned above, and the conductive shield is maintained at ground potential. By this arrangement, it is possible to energize the corona wire to a potential substantially in excess of the corona threshold potential of the wire and, since the shield is maintained at ground potential, most of the corona current emitted goes directly to the shield and only a small portion thereof is elfective to charge the plate. Thus, small differences in corona wire diameter, or accumulations of dust on the wire, or changes in ambient air conditions will have relatively little efiect on the corona current delivered to the plate since the proportionate change in the total current for a given wire diameter change is comparatively.negligible when the corona potential is maintained at a relatively high level. Such a concept is originally disclosed in my co-pending application S. N. 399,046, filed December 18, 1953, now

abandoned, of which this application is a continuationin-part.

Heretofore, a corona generating device of comparable construction has been employed for neutralizing static electricity in the manufacture of textiles. Such an apparatus is disclosed in Chapman Patent No. 940,430, issued November 16, 1909, wherein a heavy metal rod having a plurality of projecting points or needles is supported in a conductive sheath and is energized from a high voltage source to emit ions that travel to remotely located strands of yarn to neutralize opposite electrical charges thereon. However, it should be noted that a device of this sort would at best be limited to neutralizing charges already existing on the yarn. At the relatively great distances contemplated in the patent, a high charge on the yarn would probably be effective to form a sufficient electrical field to pull positive ions from the generating device n o he ya n to neutralize its charge. However, because of the relative immobility of positive tive ions generated by the corona ,device would not be directed to the yarn but would wander, inj space and eventually float to ground or recombine with'electrons to form neutral atoms, t

Preferred embodiments of the invention. are, shown in the accompanying drawings, in which:

7 Fig. 1 is a view in side elevation of a corona charging device constructed in accordance with the invention;

Fig. 2 is a bottom; view of the same;

. 2,886,725 V V i 43 discharge approximately the same as in the Fig.

modification.

Of the several. forms of corona .shield, those shown 7 in Figs. 5 and 6 are preferred for general use and may Fig. 3is a partial plan view ofone end of the device ofFigl; V U

Fig. 4 is a cross-sectional .viewtaken on line .4..4

of Fig. l, and

Figs. 5, 6 and 7. are

similar cross-sectional views of f grounded conducting shield or housing preferably of aluminum orstainless steel. The shield is of generally inverted U-shaped cross-section and includes a top wall fo'rms of wire shielding devices that may be employed I in lieu of that shown in Fig. 1.

1 and 'side wallsZ in perpendicular relation to the top wall 1.

. In the form of the invention illustrated in-Fig. 4, the side walls 2 terminate in converging portions 3, each arranged at an angle of approximately 4 5 to its side wall 2 and spaced apart to afiord a corona discharge opening or outlet 4 at the bottom approximatelyone: half inch in width, while 5 designates a high voltage wire of any suitable ,noncorrosive material such'as stainless steel having a uniform exterior and a diameterof approximatelythree and one-half thousandthsof an inch, the wire 5 .being located approximately seven-sixteenths of an inch from each of the side walls and not less than approximately seven-sixteenths of an inch from said top wall.

The high v'oltagevwire 5 is stretched between and attached to blocks 6 and 7 of suitable insulating material which are arranged between the side walls 2 andv attachedat the ends of the shield by means of suitable ion, thev insulating block 7 having attached thereto a conducting finger 9 to which'one end of the 'high voltage wire 5 is attached at 10, as shown, the finger 9 being disposed for engagement with a suitable conductingbar or source carrying the high voltage supply, asusnal in this'class of device. The opposite end of ,the conducting wire 5' is attached to insulating block '6 by a pin tained, and Figs; 5 to 7, inclusive, show modified forms of shield. Referring to Fig. 5, the converging portions indicated at 15 are disposed at a lesser angle to the side walls 2, and afford a somewhat wider opening for the corona discharge. In Fig.16,'the side walls Z-extend downwardly a greater distance than in the structures *of Figs. 4 and 5 and terminate in flanges extending inwardly at right angles to the side walls, as indicated at 16, aifording,an outlet for the corona discharge ap- I proximately the same-as in the modification shown in Fig. 5. In the structure shown in Fig. 7, the sidewalls 2 extend downwardly approximately the same distance -as in the Fig.6 modification and. terminate in converg- .ing portions' 1 7 which afford anoutlet for the :corona beused to establish similar potentials on a plate. That shown in Fig. 4 would be used with relatively high corona potentials and relatively slow plate speeds, while Fig. 7 would be used in probably the same conditions but would require greater spacing between the device and the plate? Figs. 5 and 6 differ only in constructional detailsrand would give the same elfects when operated at the same corona potentials, plate speed, and spacing, and these would normally be used with either lower 'corona poten-' tials. or higher plate speeds, or both; v With these constructions, there canbe imposed on the high voltage wire 5 a DC. current of from-7000 to 8000 volts, resulting 'in a uniform charge on the ,xero: graphic plate or transfer paper both endwise and crosswise offrom 500 to 600 volts, and the cross-sectional area of the shield acts to limit the charge on the xerographic plate or transfer sheet and to give a more nearly uniform charge both overall and in restricted areas'for diiferent voltages and with both negative and positive corona.

The structure is. simpler than prior types of charging devices employed for the'purpose, is considerably more eflicient .as to uniformity of charge over the surface of a plateor transfer sheet, and is more ruggedfand durable, while at the same timeless likely to have its efliciency affected by continued usage or dirt factorsresulting from contact with .electroscopic powder employed in developing electrostatic images.

As described above, a preferred diameter: for the corona wire is .0035" but the wire diameter isnot necests fastening screws 8 extending through the top-wall and 7 into, the insulating blocks, or in any other. suitable fash-.

sarily destricted to that dimension, since smaller Wires can be used and these will produce the same corona current at lower corona potentials. Likewise, largerdi ameter wires can be used, but an upper limit would probably. be approximately .0010", since the increase in corona potential needed to produce the required corona current in larger wires would be increased to the: point where arcing could occur, and, of a course, wouldirequire a heavier, mor expensive, power supply to produce the higher potentials needed. Similarly, ,the cross-sectional structure of-the corona Wire shieldtisl not necessarily limited to a rectangular configuration; fSuch-La form is preferred from the standpointiof ease of ma'nufacture andassembly. Obviously, a circular or similar' configuration can be employed, so long as it. permits suspending the corona wire substantially along its'long'itudinal axis, and is provided with a longitudinal .slit

of such width to prevent excessive positive'iondeposition' on the plate in accordance with the. speed and spacing. of the plate relative to the slit during a charging operation.

Thus, while the invention has been described with refer- V ence to the. structures disclosed herein, it is not confined to the details set forth, andthis'application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claimed. is: v 1. In the art of xerography wherein the photoconductivesurface of a xerographic plate is sensitized .by

the application of a uniform electrostatic charge from an ion generating device arranged in closely spaced .relation to the xerographic plateand wherein theion gen- .erating device and the xerographic platear'earranged for relative movement, a corona generating device for applying such charge comprising a, single-.corona discharge electrode of substantially. uniform circular crosssection, said single electrode being arranged for mounting parallel'to theplate surfaceand perpendicular to the path of relative movement of the plate and the electrode, a conductive shield substantially surrounding the discharge electrode, insulating means for supporting the asearcs electrode in insulated relation to the conductive shield,

mit ions emitted from the corona sheath of electrode to be directed to a xerographic plate.

tive surface of a Xerographic plate is sensitized by the application of a uniform electrostatic charge from an tion to the Xerographic plate and wherein the ion generating device and the xeroe aohic plate are arranged for relative movement, a corona corona discharge electrode of substantially uniform circular crossing parallel to the plate surface and perpendicular to the path of relative movement of the c ue the the discharge electrode, means for 21g the electrode to a source of high potential, i

conductive shield, and a longitudinal opening in the shield oriented to permit ions emitted from the corona sheath 3. in the art of Xerography wherein the photoconthe application of a uniform electrostatic charge from an ion generating device arranged in closely spaced relaerating device and the Xerographic plate are arranged for relative movement, a corona generating device for apelectrode of substantially uniform circular cross-section and having a diameter less than .010, said single elecand a longitudinal opening in the shield oriented to per- 2. In the art of xerography wherein the photoconducion generating evice arranged in closely spaced reladevice for applying such charge comprising a sin section, said single electrode being arranged for mount- L p electrode, a conductive Shield sub tan wounding S1 supporting the electrode in insulaed relation to the of the electrode to be directed to the Xerographic plate.

ductive surface of a Xerographic plate is sen 'tized by tion to the Xerographic plate and wherein the ion genplying such charge comprising a single corona discharge trode being arranged for mounting parallel to the plate surface and perpendicular to the path of relative movement of the plate and the electrode, a conductive shield substantially surrounding the discharge electrode, means for coni'iectlnr the electrode to a source or" high potential, insulating means for supporting the electrode in insulat d relation to the conductive shield, and a longitudinal opening in the shield oriented to permit ions emitted from the corona sheath of the electrode to be directed to the Xerographic plate.

4. n the art f Xerography wherein a uniform eleccharge is applied to a surface by means of an generating device arranged in closely spaced relation to the surface wherein the ion generating device and surface are arranged for relative movement, a corona generating device for applying such charge comsingle corona discharge electrode of substanuniform circular cross-section and having a diameter less than .910", said single electrode being arranged for mounting parallel to the surface and perpendicular to the path of relative movement of the surface and the electrode, a conductive shield substantially surrounding the discharge electrode, means for connecting the electrode to a source of high potential, insulating means for supporting the electrode in insulated relation to the conductive shield, and a longitudinal opening in the shield oriented to permit ions emitted from the corona sheath of the electrode to he directed to the surface.